One serious problem in the microwave re-heating of food products is the lack of crispness and browning of products which are reheated in that manner. Browning and crisping are particularly desirable when heating, bread, batter-based baked products or battered food products such as battered fish or meat. The problem with this lack of crispness and browning is directly related to the temperature which can be obtained on the surface of the product during microwave heating and to the moisture transfer within the product. The surface temperature will depend on number of factors.
Firstly, the air in the microwave oven is not heated, and is at a temperature which is slightly higher than ambient. When the product heats up, water vapor is driven out of the product. As the product surface is at or near ambient temperature, the water vapor will condense upon it and moisten it. In order to dry out the surface of the product, more microwave heating is necessary which will result in an over heating if not a burning of the core of the product.
Secondly, the microwave focusing effect due to the microwave penetration depth and the size of the product generates hot spots in the interior of the product heated in such ovens. The resulting temperature gradients which drive the moisture from the hot spot i.e. from the interior to the exterior, are thus inverse to what is required to dry out the surface.
A conventional way to solve the problem of lack of crisping and browning is to use what is known as susceptors. A susceptor is a device which comprises material that, when subjected to microwaves, absorbs energy and is heated to 200.degree. C. to 450.degree. C. (400.degree. F. to 800.degree. F.) Traditionally, the susceptor is positioned beneath and in close contact with the product to be heated and provides, by conduction or radiation, a crisping or browning of the product surface. A susceptor may e.g. be a plastic or paper substrate in which a thin layer of metal particles is deposited. Use of susceptors is however for many purposes undesirable, for example, it only allows surfaces directly adjacent to the susceptor to be crisped or browned. Also, the use of susceptors introduces a risk of migration of volatile gases into the food product, and it adds to the cost of the product.
Alternative microwave browning technology relates to emulsions which are added onto the product prior to it being microwave heated, and which e.g. are based on Maillard reactions or agents that change color when heated. Although such browning agents may be suitable for the browning of a product, they do not add to the crispness as they do not substantially change the micro-wave heating regime. Browning agents of this type in the prior art are emulsions and have a dispersed phase with droplet of e.g. 0.1 mm.